Method and apparatus for drilling



March 19, 1968 J. M. CLEARY METHOD AND APPARATUS FOR DRILLING 2 Sheets-Sheet 1 Filed Aug. 13, 1964 P o .9 200m o 0 0 6 1 I 0 u 0 Fig 5a- INVENTOR.

James M. Cleory BYMJM Atforney March 19, 1968 Filed Aug. 13, 1964 J. M. CLEARY METHOD AND APPARATUS FOR DRILLING 2 Sheets-Sheet 2 Fig. 3

INVENTOR.

James M. Cleury Attorney United States Patent 3,373,828 METHOD AND APPARATUS FOR DRILLING James M. Cleary, Dallas, Tex., assignor to Atlantic Richfield Company, a corporation of Pennsylvania Continuation-impart of application Ser. No. 79,740,

Dec. 30, 1960. This application Aug. 13, 1964, Ser.

34 Claims. (Cl. 175-65) This invention is a continuation-in-part of copending application Ser. No. 79,740, filed Dec. 30, 1960, by the same inventor and owned, by the same assignee, which application is now abandoned.

The present invention relates to a method, composition and apparatus for drilling at least a portion of a borehole in the earth. In a more specific aspect, the present invention relates to an improved method, composition and apparatus which permits running casing in a borehole during the progress of a drilling operation.

It has heretofore been the practice in the art of drilling boreholes in the earth to suspend a drilling bit from a small conduit or drill string and by rotary and/or percussive action drill a hole of the desired diameer. During the course of such a drilling operation, fluids are circulated down the borehole to the vicinity of the bit in order to remove cuttings from the bottom of the borehole and return the same to the surface of the earth and to prevent fluids under pressure from entering the hole. Such fluids may consist of a liquid such as water or oil with suitable additives or a gas such as air. In the former case, the drilling fluid serves the additional function of lubricating the drill bit, sealing the borehole against loss of drilling fluid into the formations forming the walls of the borehole and conditioning the walls of the borehole to prevent caving of incompetent sections of the borehole wall.

Where liquid drilling fluids are employed, there are a number of problems which have not been satisfactorily solved. Although most drilling fluids of this character are treated with additives to weight the fluid and to seal the walls of the borehole against drilling fluid loss and condition the borehole to minimize caving, it is not possible to fully solve these problems by the use of drilling fluid additives since the use of such additives in the amounts which would be necessary to serve this function render the fluid an undesirable one for the purpose of lubricating the bit and removing cuttings to the surface. In addition, these additives reduce the efliciency of the drilling fluid and it would be desirable to circulate a drilling fluid without these additives.

Where a gaseous drilling fluid is employed, one problem is that small amounts of water enter the well bore from formations penetrated by the well bore and make it diflicult for the gas to remove earth cuttings to the surface. This drastically reduces the ability of the bit to penetrate the formations being drilled. Such interference with the drilling operation is caused by wetting of the cuttings and the agglomeration of such cuttings beneath and around the bit. Another problem is that larger volumes of water or liquid hydrocarbons may enter the well bore partially filling the hole and making gas drilling impossible. Such entry of large volumes of liquid can also result in these fluids blowing out at the surface unless inordinately high gas pressures are used for control.

After the well bore has been drilled to total depth, it is conventional to case the borehole with tubular casing or pipe and cement the casing in place. Obviously, casing the borehole serves to ultimately solve the problems of ingress of unwanted fluids and caving of the formation walls, but the placement of casing in the borehole does not solve these problems until after the hole has been completed.

In view of the above, it would be ideal if casing could be placed in the well as drilling of the well progresses rather than after the well has been drilled. This has previously been suggested; however, no satisfactory method or apparatus has heretobefore been proposed because, first, no one has provided a safe and practical way of sealing the annular space behind the casing from the borehole below the casing; and, second, no one has provided an economical means which is removable through the casing for drilling a borehole of uniform size, which size is substantially larger than the outside diameter of the casing.

As to this latter problem, it has been proposed to use solid, one-piece bits which will drill a hole larger than the bit, but so far all of the suggested devices have proven unsatisfactory because the previously suggested bits either drilled an irregular sized borehole or drilled a hole only slightly larger than the bit. The best solution to date has been to use expansible underreaming devices. These expansible bits or reamers are run through the casing, expanded, and the drilling progresses with the casing being run behind the expansible device. This procedure is disadvantageous in that expansible devices have poor efficiency and penetration rates, are prone to malfunction and are considered uneconomical for extensive use in deep well drilling.

As to the problem of sealing the annular space behind the casing near the lower end thereof, two systems have been proposed. In one, at least the lower three feet of the casing is enlarged until the enlarged portion of the casing fits snugly in the borehole and a gelled liquid is placed in the annulus. The resistance to fluid flow between this long, enlarged, close-fitting end of the casing and walls of the borehole provides the seal. This system is totally unsuited to use in consolidated formations since the long, enlarged, close-fitting end of the casing quickly sticks and jams; moreover, as a practical matter, it is virtually impossible to drill a borehole having a diameter uniform enough to use this type of sealing device. Other solutions to this problem have suggested placing a yieldable packer element around the outside of the bottom section of casing to separate the annular space between the walls of the borehole and the outside of the casing from the well bore. This is unsatisfactory since the sealing elements of the packer wear so rapidly that the seal is lost after the packer has travelled a short distance through the borehole. When the packer seal element is damaged, the annular seal between the casing and walls of the borehole is lost and the fluid inside the casing and the fluid outside the casing mix. If the fluids in the borehole are unable to control the well, a blowout will occur. Even if the packer elements are replaced at frequent intervals of about every 300 feet or less, the reliability of this packer seal is too risky to permit regular use of the packer system. In addition, it is expensive and time consuming to pull casing frequently to replace packer sealing elements. There are many other disadvantages to the packer sealing system just described.

It is, therefore, an object of the present invention to provide an improved method and apparatus for drilling boreholes in the earth.

Another object of the present invention is to provide an improved method and apparatus which permits placement of casing in a borehole as drilling of the hole progresses.

Still another object of the present invention is to provide a method and apparatus for shutting off fluid ingress during air or gas drilling operations.

Another and further object of the present invention is to provide an improved method and apparatus which permits the maintenance of two fluids of different character in the borehole during the course of a drilling operation.

'Still another object of the present invention is to provide an improved method and apparatus which enables one to run casing in a borehole during gas drilling operations and thereby greatly extend the range of conditions under which such drilling can be utilized.

A still further object of the present invention is to provide a novel apparatus and composition which coact to form a mobile particle seal between the annular space surrounding a casing and the well bore when casing is run behind a drill bit during drilling.

Another and further object of the present invention is to provide a behind-the-casing fluid admixture which forms a mobile particle seal between the casing and the wall of a bore hole so as to prevent the passage of fluid from behind the casing to the well bore when running casing while drilling.

These and other objects of the present invention will be apparent from the following detailed description when read in conjunction with the drawings.

In the drawings:

FIGURE 1 illustrates, partially in section, the apparatus and method of the preesnt invention when utilizing a drilling means which drills a borehole substantially larger than the outside diameter of the casing.

FIGURES 2 and 3 show sectional views of a filter sealing element for use with this invention.

FIGURE 4 shows a fragmented sectional view of the filter sealing element of FIGURES 2 and 3 with a circular cutting edge on the lower end thereof.

FIGURES 5a and 5b show a fragmented sectional view of a resilient end for the filter sealing element of FIGURE 2.

FIGURE 6 shows an auxiliary sealing means.

Briefly, the present invention concerns running casing while drilling an earth borehole and is directed to methods, apparatus and compositions for forming a novel seal between the wall of the borehole and the casing. This seal follows movement of the casing as the casing is advanced into the borehole and prevents the flow of fluids between the casing-borehole annular space and the open borehole below the casing or interior of the casing. The seal is composed of an unconsolidated, fluid-impermeable mass of particle-form solids. This invention sets forth the properties of a flowable admixture of a gelled, highly viscous nonaqueous liquid and particle-form solids, which admixture is especially suited to forming the aforementioned particle seal and is especially designed for use hehind the casing while running casing during drilling. The invention provides a method of drilling employing this admixture while running casing during drilling. The invention also covers apparatus for drilling and running casing. In one embodiment the apparatus pertains to a casing having an enlarged circular cutting edge near the lower end of the casing, and a lateral projection surrounding the casing above this cutting edge. The lateral projection is combined with the particle-form seal previously mentioned. In a second embodiment, above the circular edge, is a filter sealing element which has flow passages which allow liquid and small particles in the admixture to leak past the filter sealing element to build a bridge of larger particles and eventually form the particle seal. In a third embodiment, a pressure differential actuated sealing means is placed above the particle seal to prevent downward flow of liquid in the casing-borehole annular space whenever downward flow of the admixture would otherwise exceed a predetermined rate, which excessive rate would occur if the particle seal was lost. A fourth embodiment covers a novel drill bit which when used in combination with the other apparatus provides a complete and highly efficient drilling system. This drill bit has a negatively sloping noncutting surface on an eccentric pilot cutting portion which causes the drill bit to drill a hole substantially larger than the casing. The position of the noncutting surface, length of the pilot and a cutter on the pilot are arranged in a manner which causes the bit to drill a very uniform gauge borehole. In the aforementioned process, a large, uniform-size borehole is necessary. Other embodiments cover specific features of the apparatus and two or more filter sealing elements longitudinally spaced from one another.

Additional process claims cover drilling a borehole larger than the casing and running the casing either intermittently or continuously behind the drill bit. While the hole is being drilled, a first drilling fluid is circulated through the interior of the casing and a drill string to remove cuttings. This first drilling fluid may have a much lower specific gravity than the behind-the-casing fluid admixture previously mentioned, and may even be a gas. During the period when running casing while drilling, a particle seal is used to prevent excessive flow of admixture from behind the casing to the open borehole or to the interior of the casing. In general, the lower end of the casing will have an enlarged circular cutting edge which will be used to cut inwardly extending projections from the walls of the borehole.

The above-mentioned embodiments and this invention are illustrated in more detail by reference to the drawings, where in FIGURE 1, there is shown borehole 11 being drilled in the earth. The drilling of the borehole is accomplished by rotary-percussive type bit 13 suspended from drill string 15 through percussive unit 17. Bit 13 is designed to drill the ultimate diameter of the borehole and yet pass through a more restricted opening than one equal to the diameter of the borehole. This may be accomplished by providing an expansible and contractable bit or reamer which may be lowered into the borehole and expanded to accomplish drilling. When it is desired to replace the bit or reamer or remove it from the borehole, drilling is stopped, the bit or reamer is contacted and then pulled from the borehole. Bits and reamers of this character are, however, restricted in use to short sections of a borehole since they have limited speeds of penetration and are subject to frequent breakage and malfunctioning. A more ideal solution is to use the unitary type of underdrilling bits described in copending applications Ser. No. 79,687, filed Dec. 30, 1960; Ser. No. 79,738, filed Dec. 30, 1960; and Ser. No. 79,737, filed Dec. 30, 1960; all filed by the same inventor and owned by the same assignee as the present application. As described in these applications and as shown in FIGURE 1, the rotary bit has main bit body 19 with rotary pilot drilling means 21 having a vertical central axis offset from the central axis of the main bit body. On the side of the pilot drilling means farthest from the axis of the main bit body is sloping noncutting surface 23. The slope of this noncutting surface depends on the direction of the main drilling or cutting force of the cutters on the bit and is adapted to force the pilot drilling means away from the wall of the borehole and inwardly toward the center of the borehole. This is accomplished because the sloping noncutting surface extends beyond the cutting edges of the pilot and resists digging or cutting by the cutters on the pilot. In other words, the noncutting surface slopes outwardly from the center of the bit and forwardly with relation to the direction of the main drilling force of the cutters on the pilot. In the drilling art, this is called a negative relief angle or negative slope. The bit shown in FIGURE 1 is a percussive bit and the main cutting direction of the bit is downward; therefore, as shown, noncutting surfaces 23 slopes inwardly toward the axis of the central main bit body and forwardly with respect to the direction of penetration of the bit. On that portion of the pilot drilling means nearest the central axis of the main bit body extending from adjacent the central axis of the pilot drilling means to the inner edge of the pilot drilling means is at least one first cutting means 25. Cutting means 25 is adapted to cut a cylindrical pilot hole whose circumference accommodates the outermost extension of noncutting surface 23 when the central axis of the rotary pilot drilling means concides with the central axis of the borehole. Above the cutters on the pilot located on that side of the main bit body opposite the pilot drilling means and trailing said noncutting surface with respect to the direction of penetration of the bit by a distance of between 0.15 to 1.5 times the maximum width of the bit is at least one second cutting means 27, commonly called a gauge cutter. Second cutting means 27 is adapted to cut an annular section of the earth immediately surrounding the pilot hole drilled by the pilot drilling means and trailing the pilot hole with respect to the direction of penetration of the bit. The bit just described will cut a borehole having a diameter at least one-tenth larger than the outside diameter of casing 29 which is lowered behind bit 13 as hereinafter described. The diameter of the borehole drilled with the bit will usually be at least one-half inch larger than the outside diameter of the casing.

In order to remove cuttings to the surface of the earth, kept the drill bit clean and prevent the intrusion of fluids under pressure, a suitable first drilling fluid is circulated down drill string 15 out the bottom of bit 13 and back to the surface of the earth through casing-drill string annular space 31 between drill string 15 and casing 29. The particular first drilling fluid employed in this instance is designed primarily for the removal of drill cuttings and keeping the bit clean. Thus, this first fluid can be nonviscous and contain little or none of the conventional additives which are employed to control loss of fluid through the borehole walls, build a filter cake on the face of the formation and perform other functions which are generally required of the normal drilling flud. Preferably, the circulated fluid will have a specific gravity less than hereinafter described behind-the casing fluid admixture and could be gas or air. It is possible to utilize a drilling fluid of this character because the drill bit is followed by casing 29 which controls the previously drilled formations. For example, the casing prevents loss of drilling fluid through the walls of the borehole above the bit, prevents incompetent formations from caving into the hole and prevents the ingress of formation fluids. Thus, the casing may accomplish many controls which have heretofore been left to additives in conventional drill fluids which additives were generally detrimental to the actual drilling or cutting operation and the removal of cuttings from the hole.

Since fluids which are best suited to the drilling are not well suited to sealing of the formation walls, control of ingress of formation fluids and the like, the drilling fluid circulated through casing-drill string annular space 31 is not suited for use behind casing 29 in casing-borehole annulus 33 between the walls of the borehole and the casing. Accordingly, a fluid of entirely different character is disposed in the casing borehole annulus behind casing 29. This fluid is herein called the behind-the-casing fluid and is an important feature of this invention. This behind-the-casing fluid will be hereinafter described in greater detail, but it should be pointed out now that this is an admixture of a gelled, highly viscous, nonaqueous liquid and particleform solids of graded size and that it is the particles in this admixture which form a mobile particle seal which seal separates casing-borehole annulus 33 from casingdrill string annular space 31 and the open part of borehole 11. In FIGURE 1, this seal is shown by the pack of particles near the lower end of casing 29.

Near the lower end of casing 29, there is outward lateral projection 35 which surrounds the casing. Projection 35 will extend at least one-eighth inch beyond the outer surface of the casing. Lateral projection 35 is adapted to initiate the particle seal. The outside diameter of projection 35 should be at least equal to the interal diameter of borehole 11 minus one-tenth inch. In some cases, when the projection is resilient, the unrestrained outside diameter of resilient projection will be greater than the diameter of the borehole. A resilient projection 6: is shown in FIGURES 5a and 5b and will be hereinafter described in more detail. Projection 35 should be constructed of a high wear resistant material.

As stated previously, lateral projection 35 is adapted to initiate the particle seal. The high pressure side of this projection drains to the low pressure side. The projection, therefore, provides a flow restriction and drainage system to initiate the formation of the particle seal. This may be accomplished by providing clearance between the outer edge of the projection and the wall of the borehole; or by providing a liquid passage through the projection; or by providing a liquid flow passage around the projection; or by a combination of these. Generally, the flow passage around or through the projection will be large enough to pass particles 0.03 inch in size for reasons hereinafter made apparent.

As shown in FIGURES 1 through 3, lateral projection 35 is part of filter sealing element 37 which acts as an aid for forming the particle seal, for controlling the vertical length of the particle seal, and for preventing excessive consolidation of the particles in the particle bridge. 7

Filter sealing element 37 has flow passages which pass particles of at least 0.03 inch, but which retain particles greater than 0.2 inch. Within this range of 0.03 to 0.2 inch, the size of the openings of the flow passages of the filter sealing element will be varied to pass all particles less than a predetermined size and retain all particles greater than this predetermined size, which predetermined size will be determined by the size of the particles in the behind-the-casing admixture which forms the particle seal.

As shown in FIGURES 2 and 3, the openings in the filter sealing element are formed by a series of slots 39 which traverse all but top part 41 of the filter sealing element thereby forming ribs 43. As shown the slots also pass through projection 35; however, it is not necessary that projection 35 be slotted. The number and the width of the openings of slots will depend on the particle sizes in the admixture and on strength considerations. For illustration purposes, there are only eight slots shown for the filter sealing element of FIGURE 3, but the number of slots will normally be much higher and may even be as high as 60 or more. In the drawings, the size of the slots and the number of slots were selected to illustrate the features of the filter sealing element and are not drawn to scale.

As shown in FIGURE 1, filter sealing element 37 surrounds casing 29 and is located near the lower end of the casing. As will be described, the slots in the filter sealing element are adapted to pass all particles in the admixture except those that are above a predetermined size. This is important to the formation of the particle seal. During drilling the particle seal may need to reform itself any number of times. It is, therefore, important that slots 39 remain open. It was found that the desired leak off through the slots could be better maintained if the walls of the slots were tapered with the narrowest portion of the slot at the outside surface of the sealing element at the point where the slot contacts the sealing admixture. In this manner, any particle passing a slot will be sure to pass through the remainder of the slot and be drained to the low pressure side of projection 35. For example, there may be sixty dovetail slots inch wide at the outside surface, opening gradually to 1; inch at the inside next to the casing.

The liquid filtrate with small particles that has passed through the openings of the slots must be allowed to flow to the lower pressure open part of the borehole. This flow is facilitated by providing filter seal annular space 45 between casing 29 and the filter sealing element. As shown in FIGURES 1 and 3, this annular space is formed by making the filter sealing element larger than the casing. The annular space is held open 'by top part 41, and by longitudinally spaced ridges 47 on the interior of the filter sealing element. These ridges rest against the casing and hold the slotted portion of the filter sealing element away from the casing, thereby providing the desired annular flow space, which space conducts the liquid filtrate with small particles passing through slots 39 to the low pressure side of the particle seal. As shown, ridges 47 are aligned, but the ridges could be staggered to improve fluid flow should this be needed. Filter seal annular space 45 may be Vented directly to the inside of the casing if desired.

Ribs 43 extend upward from projection 35 by a distance x which is greater than the usual vertical length of any cavity encountered in the side of the borehole. The length x must be at least two inches, and more preferably, will be a foot or more in length, but should not be so long that particles deposited on the ribs and slots will impede movement of the casing or cause sticking thereof. There should be enough clearance between the outer surface of the ribs and the boreholes to allow the largest particles in the behind-the-casing fluid admixture to travel downward to projection 35 where the particle bridge is initiated.

It has also been found that the outer surface of ribs 43 contacting the particle seal should be scored as shown in FIGURE 2 by grooves 49. These grooves help to sup port the particles and prevent excessive consolidation or compacting of the particles in the particle seal and cause a more uniform pressure and stress gradient through the pack of particles in the particle seal. The effect of grooves 49 on consolidation or compacting of the particles was manifested by a test in which the same behind-the-casing fluid admixture formed a 4.25-inch bridge with a scored surface while with a smooth surface, the bridge was 1.2 inches long. If filter sealing element 37 is not used to control the length of the particle bridge, the degree to which the particles will compact would greatly affect the length of the particle bridge. Regardless of whether or not the filter sealing element is used, it is desirable to prevent overconsolidation of particles in the particle seal because overconsolidation may impede'movement of the casing. Overconsolidation also causes plugging of the filter sealing element so that the particle seal will not readily reform itself should the seal be broken as the casing is advanced into the borehole. Grooves 4-9 have the added advantage of aiding leak off of the liquid filtrate passing through the large-size particles retained on the filter sealing element as the particle is formed.

As shown in FIGURE 2, projection 35 is made up of three vertically-spaced slotted disc-shaped rings. This improves reliability of the projection in initiating the seal and at the same time prevents stickingof the casing. This projection should extend at least 0.125 inch, and preferably further, or 0.25 inch, beyond the outside surface of vertical ribs 43 of the filter sealing element so that the projection forms a definite point for the particle seal and permits sufficient clearance for the largest particles in the admixture to reach the shoulder formed by the projection. The particle bridge extends upward from this shoulder.

An alternative arrangement for projection 35 is shown in FIGURE 5 by projection 35' wherein a series of radially-spaced resilient wires or bands 51 are used in place of rigid projection 35. As shown, bands 51 are closelyspaced bow springs. Bands 51, being resilient, may extend outward from the casing and have a normal outside diameter greater than the diameter of the borehole. In this manner, if the filter sealing element passes through an enlarged portion of the borehole, the resilient members can expand to help maintain the particle seal. Resilient members have the added advantage of being more wear resistant than projection 35.

Bands 51 are spaced to provide the slots for leak off of liquid as previously described. The bands may be held in place by any suitable means; for example, the bands may be rigidly held in place by straps 53.

Because the problems caused by eccentricities in borehole size, higher pressure differentials, and weak or rotten shale sections increase when drilling deep wells, there should be a minimum of two longitudinally spaced points at which the particle seal may be formed. These points should be at least three feet apart. A preferred arrangement is comprised of three filter sealing elements spaced at five-foot intervals with the lower element placed a few inches above circular cutting edge 55, hereinafter described in greater detail. This three-seal arrangement provides at least one sealing element in a gauge section of the borehole and makes it more diflicult for a hole enlargement to envelop the entire particle seal. Multiple elements also reduce the pressure drop and wear on the filter seal and particles, and render the filter sealing system more clog resistant.

The filter sealing element is especially suited to use with the preferred behind-the-casing fluid admixture hereinafter described in detail.

It has also been found that the reliability and safety of the novel particle sealing system of this invention may be improved by providing a pressure differential actuated sealing means in the annular space behind the casing. This pressure differential actuated means is adapted to seal the annulus between the casing and borehole whenever the particle seal is lost and flow of behind-the-casing fluid down the annulus exceeds a predetermined rate. Preferably, the pressure differential actuated means should open when the particle seal reforms. A pressure differential actuated means of this type is shown in FIGURE 6, wherein helical spring 57 surrounds casing 29. The lower end of the helical opening is fixed to retaining ring 59 and the upper or top end is free. Above the retaining ring, the spring slides freely up and down on the casing in compression and extension. When extended, there is ample room for the behind-the-casing fluid to flow down around the casing between the spiralling members of the spring and past retaining ring 59. But any flow in this helical passage tends to compress the spring and close the passage. The force tending to close a given turn in the spring is equal to the fluid pressure drop in the coils or helical passage above it; therefore, the lowest turn of the helical spring will close first sealing the helical flow passage. If this stops the downward flow of behind-thecasing fluid, the other turns of the spring will not collapse or compress. Unless the borehole is enlarged opposite the collapsed lower coil, the particles in the admixture will form a particle seal over the clearance between the outer edge of the collapsed coil and the wall of the borehole. Since there is no porous sealing element, the particle bridge will be short. This short seal is subject to leakage and will break and reform as the casing is moved, thereby allowing a small intermittent amount of particles and admixture to spurt past the retaining ring. The particles and admixture passing the retaining ring will reform the primary seal on the filter sealing element when the condition causing loss of the primary particle seal has been corrected. When the primary seal is reformed, the helical spring will extend, reopening the helical passage.

Referring now to FIGURES 1 and 4, there is shown circular cutting edge 55 which is located on the casing below projection 35 and which moves downwardly with the casing. Usually, projection 35 will be within twelve inches of the circular cutting edge. Circular cutting edge 55 has an outside diameter substantially equal to the desired diameter of borehole 11 and cuts inwardly extending projections from the walls of the borehole. This assures more uniform borehole diameter and improves the action of the particle seal. The cutting edge should be wear resistant and fabricated of a hard metal like sintered tungsten carbide.

As shown in FIGURE 1, the cutting edge is on the bottom of flared cutting shoe 61 which is attached to the bottom of the casing. In addition to cutting the walls of the borehole, this flared cutting shoe may provide an additional seal for the bottom of the casing since small particles passing through the filter sealing element may 9 be screened out between the cutting edge of the shoe and the walls of the borehole where the shoe fits tight in the borehole; however, without the seal on projection 35, this seal at the flared cutting shoe would not be sufficiently reliable.

In FIGURE 4, flared cutting shoe 61 is an integral part of the filter sealing element. When the cutting shoe is a part of the filter sealing element, it is necessary to provide flow passages for the liquid and small particles escaping through the porous filter sealing elements. As shown, the escape passages are provided by outlet holes 63 which communicate with the annular space between the casing and the filter sealing element and conduct fluid flowing down this annular space outward where the fluid will leak around the cutting edge screening out the small particles forming the auxiliary seal just mentioned.

The cutting shoe as illustrated in FIGURES 1 and 4 is designed to help maintain a borehole that is at least one-half inch larger in diameter than the outside diameter of the casing. The cutting shoe is shown to have horizontal angle 65 of 15 degrees, but this angle maybe as small as degree. It should be noted that above the cutting edge, the outer surface of the cutting shoe should be relieved by angle 67 at least 1 degree and preferably degrees. This rel-iefangle should be large enough to allow some of the small particles passing through the filter sealing element to collect between the cutting shoe and the wall of the borehole. The relief angle may be less near the cutting edge to reduce wear on the cutting edge, but the outer surface of the cutting shoe should be relieved and should not be vertical for more than two inches since a longer distance would cause sticking of the casing or excessive wear of the cutting shoe.

The properties of the behind-the-casing fluid admixture will be better understood by first referring to the following description of the method of drilling wherein this admixture is used to form a fluid-tight seal between the casing and wall of the borehole.

In accordance with the drilling method of this invention, drill bit '13 is rotated, or rotated and vibrated by way of tubular drill string to drill borehole 11 which is substantially larger than the diameter of the drill bit and which normally will have a diameter at least onehalf inch larger than the outside diameter of casing 29. The drill cuttings are removed by circulating a drilling fluid into and out of the borehole. If standard circulation is used, the drilling fluid will be circulated into and down the borehole by way of drill string 15 and up and out of the borehole by way of annular space 31. In reverse circulation, the flow of the drilling fluid is reversed. The drilling fluid may be a light liquid or a gas.

As the borehole is drilled, tubular casing 29, which has an internal diameter such that drill bit 13 may be removed through the casing, is advanced into the borehole at a predetermined distance above the drill bit. The casing may be advanced immediately behind the drill bit or the casing could be advanced intermittently in stages of several hundred feet or more.

Near the lower end of the casing, there is maintained an outward lateral projection which surrounds the casing and which is adapted to allow liquid behind the casing to leak around the projection to the open borehole or to the interior of the casing. This lateral projection will extend from the casing by a distance of at least oneeighth inch and provides a flow restriction to initiate the formation of a particle bridge and seal.

Above this lateral projection in casing-borehole annulus 33 between the walls of the borehole and the casing, there is disposed and maintained the behind-the-casing fluid admixture which forms a liquid-tight, mobile seal of particles between the casing and wall of the borehole above the lateral projection. This admixture is composed of a gelled, highly viscous nonaqueous liquid mixed with particle-form solids of graded size. The particle-form solids in this admixture are used to maintain a fluidimpermeable seal consisting of unconsolidated particles. This seal follows the casing and lateral projection as they move downward in the borehole. The seal prevents the flow of fluids from the casing-borehole annulus to the interior of the casing or the open portion of borehole 11 below the casing. Preferably, the behind-the-casing fluid admixture will have the properties hereinafter set forth and will contain particles in two distinct size groupmgs.

Preferably, below the lateral projection near the lower end of the casing, there should be maintained circular cutting edge 55 which has an outside diameter substantially equal to the diameter of borehole 11. This cutting edge is moved downward with the casing and cuts inwardly extending projections from the walls of the borehole. The weight of the casing is in general suflicient to cause this cutting edge to cut or broach these projections from the wall of the borehole. However, if the weight of the casing is not suificient, the casing may be vibrated, rotated or pushed at the surface to provide the needed cutting force. When the cutting edge is not cutting, the casing will be hung in tension.

In the above-mentioned drilling process, the behindthe-casing fluid admixture forms a fluid-tight seal of unconsolidated solid particles between the casing and wall of the borehole. This particle seal moves downward with the casing and immediately reforms itself should the seal be broken. In addition, the behind-the-casing fluid helps to maintain initial uniform borehole size, precludes flow from productive zones above the particle seal, and prevents loss of the behind-the-casing fluid to formations above the seal. To carry out these functions and perform satisfactorily, the behind-the-casing fluid admixture and particle seal should possess the following properties:

(1) The behind-the-casing fluid admixture will remain essentially stagnant behind the casing for long periods and should not form a thick filter cake on the wall of the borehole; consequently, fluid loss by filtration of liquid from the admixture through porous formations should be as low as is practical. The fluid loss of the admixture should be less than 0.1 cc. when obtained at the surface in accordance with the procedures for conventional low pressure testing of drilling fluids as described by The American Petroleum Institute in a publication entitled API RP29, Fourth Edition, May 1957, Recommended Practice, Standard Field Procedure for Testing Drilling Fluids. More preferably, the fluid loss should be less than 1 cc. at degrees Fahrenheit in thirty minutes. The lower the fluid loss, the better, and a fluid loss of 0 is sought. Generally, the fluid loss of the admixture is controlled by adding clayey-size particles and other small size particles to the liquid. Frequently, the clayey-size particles are bentonites; however, in order to improve the lubricating nature of the admixture soap-type particles may be substituted for the bentonites.

(2) The admixture should have a Brookfield viscosity of at least 1000 centipoises, and more preferably, the viscosity should be in excess of 10,000 centipoises and higher. In fact, a viscosity of 20,000 centipoises and higher is preferred provided that the admixture is still flowable. The viscosity of the admixture is important for many reasons. For example, the admixtures homogeneity must remain constant for long periods while the admixture is essentially stagnant. Also, should the seal break momentarily, or should the seal pass a portion of the borehole wherein the walls of the borehole are enlarged, it is highly desirable that the natural tendency of the admixture to spurt from behind the casing be low, that is, the resistance to flow during the time that the seal is lost should be high. The high viscosity and gel strength of the liquid admixture helps to accomplish this and the casing and seal can pass points where the seal is most likely to fail without washing out or enlarging the borehole by jet erosion or by fracturing the formation. The high viscosity and gel strength of the admixture also help to prevent undesirable consolidation of the particles and help to distribute the stress gradient and pressure drop over the particle seal. This in turn reduces wear on the particles and controls the unit shear strength of the particles lessening the tendency of the bridge to impede motion of the casing or to break down and extrude around the lateral projection at weak points in the formation or at enlarged portions of the borehole. The high viscosity of the admixture has the added advantage of reducing the length of the bridge required and reducing the amount of fluid loss from the admixture.

(3) There will normally be a large stress gradient or pressure drop across the particle seal; therefore, the strength of the particle bridge formed by the particles should be capable of withstanding pressure differentials of at least 1500 pounds per square inch; and depending upon the conditions encountered, especially when drilling with gas, the bridge may be required to withstand pressures between 1500 and 10,000 pounds per square inch and greater. The strength of the bridge is dependent upon the length of the bridge, the strength of the particles, and the stress gradient and pressure gradient through the particles. The particle bridge should withstand these pressures after the admixture has been bombed for a period of at least one day at a temperature of at least 150 degrees Fahrenheit. In general, the bridge will be strong enough and sufficiently resistant to degradation to withstand the temperatures and pressure differentials to be encountered for periods of between one and 45 days.

(4) The liquid in the admixture must be inert to the formation and should not swell or cause sloughing of the formations traversed by the liquid. In addition, the admixture and liquid in the admixture should have a high lubricating nature, thereby resisting drag by the particles and providing a lower coefiicient of friction. The liquid must, therefore, be a nonaqueous liquid or at least one wherein hydrocarbons are the external phase of the liquid so that only hydrocarbons will contact the formation and the casing. It has been found that crude oil, asphalt cut with kerosene, and other like materials are highly desirable base liquids. Water-in-oil emulsions in which oil is the external phase are also suited for use in the present invention and such emulsions are classified as nonaqueous. It has also been found that the lubricating nature of the admixture may be improved by minimizing inorganic or abrasive particles and by using soft, lubricating-type particles Where practical. For example, as stated previously, it may be possible to change the clayey or colloidal-size particles from bentonites to soap-type particles of a more lubricating nature. A suitable hydrocarbon base liquid may be formed by cutting road asphalt just enough to make it fiowable.

(5) As stated previously, the behind-the-casing fluid admixture remains essentially stagnant for a long period. Yet the admixture must retain the desired properties throughout this period. The liquid must, therefore, be treated with a gelling agent in order to render it capable of suspending the solid particles and maintaining the homogeneity of the admixture. The admixture should have a high gel strength, that is, between and 100 pounds per hundred square feet after ten minutes. A gel strength of at least 50 pounds per hundred square feet after ten minutes is preferred. It has been found that conventional tall oil soaps are an excellent material for adding sufficient gel strength to the nonaqueous liquid, This high gel strength, in combination with the extremely high viscosity of the admixture, has the advantages mentioned previously. The high gel strength also helps to maintain a uniform borehole and holds loose sections of formations in place in the walls of the boreholes.

(6) The behind-the-casing fluid should retain its properties at elevated temperatures. The geothermal gradient Will tend to change the casing fluid properties adversely in a very deep hole, since the viscosity and gel strength decrease with increasing temperatures. It has been found that the effect of temperature can be eliminated or even reversed by adding pulverized, high melting point asphalt to the mixture. The pulverized asphalt swells extracting solvents from the liquid asphalt. The solid asphalt addition thereby increases the gel strength and viscosity and decreases the leak otf rate. Swelling of the solid asphalt proceeds very slowly at ambient temperature and accelerates with increasing temperature. Thus, the pulverized asphalt will either thicken the fluid mixture with temperature or hold it constant, depending on the amount and melting point of the asphalt selected.

(7) As stated previously, the admixture should be adapted to form a particle bridge of at least two inches in length, and preferably, of at least 4 inches in length; however, the bridge should not be extensive enough to impede motion of the pipe or casing. The length of the bridge is critical in order to distribute the stress applied to the formation and to reduce the stress at the bot tom of the seal. Too high a stress at the bottom of the seal could cause the formation to fracture around the lateral projection and result in loss of this seal. In addition, the gradual change in stress and the minimized pressure gradient prevent overconsolidation of the particles, lessen extrusion of the particles around the lateral projection, and reduce jetting of the fluids thereby preventing spurt erosion and providing the time and control needed for preventing blowout or loss of control of the formations behind the casing. This also prevents excessive particle Wear and lessens the strength requirements of the particles as mentioned previously. It has previously been pointed out that the length of the bridge is best controlled by filter sealing element 37 and by the roughened outer surface of this element. As was stated previously, the roughened surface helps to support the particle bridge by increasing friction between the bridge and the filter sealing element. This also prevents compacting and excessive stress gradient through the particles. It has also been noted that the length of the bridge is partially controlled by the high gel strength and high viscosity of the liquid admixture. In addition to the aforementioned factors, it has been found that the length of the particle bridge may be controlled by dividing the particles into two groups. The first group is composed of large size particles in a narrow size range with all of the particles at least 1.25 times larger than the openings in the filter sealing element. The second group of particles is composed of small-size particles covering a broad range of sizes varying from colloidal size to the size of the largest size particle in the second group. The largest size particles in the second group must be no larger than the openings in the filter sealing element. The length of the particle bridge is controlled by adjusting the size ratio and volume concentration (grain volume) ratio between the first group and the second group. It has been found that particles having a size equal to 0.75 of the size of the largest size particles in the second group should be between 0.125 and 0.25 and never greater than 0.33 the average size of the particles in the first group. More preferably, the largest size particles in the second group will be less than 0.33 the size of the smallest particles in the first group. This wide gap in size between the first group and second group of particles causes the admixture to have a high spurt loss and form a thick bridge of group one particles on openings greater than the size of the largest particles of the second group of particles. On openings less than the size of largest particles of the second group, the sizes and concentration ratios of the particles in the second group are such that there will be a small spurt and a thin bridge formed, and through openings the size of pores in a porous formation, there is very little fluid loss.

The relative sizes and volumes of the particles in the behind-the-casing fluid admixture and the Width of the openings in the filter sealing element are set forth below using D to designate the maximumdimension or the size of a particle, C to designate grain volume, and W to represent the width of the openings in the filter sealing element. There are four particle sizes and two volumes 13 that must be defined and related in order to properly form the preferred particle seal of this invention; consequently, the symbols. D and C are accompanied by appropriate numerical subscripts which refer to the particle or group of particles being defined. When the symbol D is accompanied by a double numerical subscript, the symbol D relates to the arithmetic average of the size of the two different sized particles represented by the numerical subscripts. When the symbol C is accompanied by a double numerical subscript, the symbol C relates to the volume of the particles ranging between the particle sizes represented by the numerical subscripts. The reasons for using this system of designating and relating the sizes and volumes of the particles will become clearer after studying the following definitions and relations; however, it should first be recalled that the preferred behindthe-casing fluid admixture is composed of a first group of larger particles in a narrow size range and a second group of smaller particles in a broad size range. There is a size gap between the first group and the second group of particles, and the width of the openings in the filter sealing element comes within this gap so that the first group of particles will be retained on the filtersealing element while the particles in the second group may pass through the filter sealing element to the low pressure side of the seal. In other words, all of the particles in the first group are larger than the width of the openings of the filter sealing element while :all of the particles in the second group will pass through the openings in the filter sealing element. The first group of particles has been defined as those size particles between D and D with an average size of 0.5 (D +D and a grain volume of C As stated, the first group of particles are retained on the filter sealing element and, being in a narrow size range, (i.e., D is at least 0.5D form a highly permeable pack of particles. The second group of particles being comprised of a broad size range eventually form a fluid-tight particle seal. It was found that the second group of particles must contain sufficient particles of a size large enough to build this seal. This cut or part of the second group has been defined as those size particles betwen D and D when D equals .5D and when the average of D and D is 0.75 D The grain volume of the D to D size, particles is C The following definitions and relations set forth the relative sizes and volumes for these particles and width of the openings in the filter sealing element.

D is the size of the largest size particle in the first group which is defined as the size of the smallest size standard U.S. Sieve Series Screen opening through'which at least 98 'percent by volume of the particles will pass.

D is the size of the smallest size particle in the first group which is defined as the size of the largest size standard U.S. Sieve Series Screen opening on which at least 98 percent by volume of the first group of particles will be retained. D must be at least 0.4 the size of D to provide a narrow size range for the first group of particles.

D 3 is the average size of the particles in the first group and equals 0.5 (D +D 0 is the grain volume of particles between D and D that is, the grain volume of the first group of particles.

W is the width of openings in the filter sealing element.

D is the size of the largest size particles in the second group which is defined as the size of the smallest size standard U.S. Sieve Series Screen opening through which at least 98 percent by volume of the particles in the second group will pass.

D is equal to 0.5D

D is the average of a cut of the largest size particles in the second group and equals 0.75-D

C equals the grain volume of particles between D, and

From the above, the following relations have been found desirable when the filter sealing element is not used. Even when the filter sealing element is used, it has been found best to use a behind-the-casing fluid with these properties; however, the filter sealing element helps to overcome variations in the properties and to make the properties less critical.

D must be less than 0.1 times the outside diameter of the casing and generally less than 0.5 inch.

D must be greater than 0.06 inch and greater than 1.25W.

W must be at least 0.03 inch and less than 0.2 inch.

D must be equal to or less than W.

D must be less than about 0.33D and should be greater than 0.12SD and will usually be beteween 0.33D and 0.2D

C must be at least three times C and up to twenty times C Above three, the concentration ratio between the D particles and the 13 particles is not as critical as the relative sizes of the particles just given.

Perhaps, the following explanation will illustrate how the particle bridge length may be controlled and why the sizes and concentrations just given are important. The second group of particles is composed of particles covering the range from D down to colloidal size. When the particles are sized in a descending order with a large gap between the sizes of any particle from the next larger particle, there will be a 'low controlled spurt of fluid through openings smaller than D and the particles will be a thin impermeable bridge over the openings. In this invention, the behind-the-casing fluid must build a tall bridge of particles before the seal is formed. This is accomplished in two ways. First, the sealing element has openings large enough to pass D size particles and smaller. Second, there is provided .a relatively large volume of the large-sized particles of group one having sizes between D and D in a narrow size range. The D size particles are at least 1.25 times the openings in the fi'lter sealing element and will not pass through these openings. By sizing the D particles much larger than the D particles, the openings between the D particles deposited on the filter sealing element are large enough to pass most of the particles in the second group. Consequently, the D particles are deposited in a thick, unconsolidated bridge of high permeability. However, as the bridge of D particles increases, the available number of flow paths for passage of liquid bearing D and smaller particles through the bridge diminishes and occasionally some of the D particles are entrapped. As a result, eventually the openings through the bridge of D particles perform the same as openings less than D and the particles in the second group quickly form a fluid-tight seal, stopping flow of behind-the-casing fluid and stabilizing the bridge thickness at the desired level. It should be recalled at this point that the extremely high viscosity of the behind-the-casing fluid admixture plays an important role in the formation of this bridge; consequently, the properties of the admixture are interrelated. Since the formation of the bridge depends in part upon the entrapment of D size particles, the concentration of these particles is important. If the concentration C is too great the bridge will not be thick enough, and, if the concentration is too small, the bridge will grow until the restriction to fluid flow through the growing pack reaches a point where the high viscosity fluid can no longer pass through the bridge. As stated previously, this is controlled by varying the concentration ratio of C to C between 3 and 20. The upper limit on the ratio will not be as critical when the viscosity of the liquid and the pressure drop across the pack are such that the length of the bridge will not stick or impede motion of the casing.

(8) The particulated materials used in the behind-thecasing fluid admixture should be temperature resistant and inert to the fluids encountered in the borehole including the base nonaqueous liquid slurry to which the particulated matter has been added, so that the desired properties of the behind-the-casing fluid will be retained throughout the drilling period when the behind-the-casing liquid is being used to form the seal between the casing and the wall of the borehole.

(9) Preferably, the large size particles in the first group of particles will be round or granular instead of being thin plates or laminar.

(10) The amount of particles required depends on the size of the largest size particles and the required length of the particle bridge. Generally, it has been found that the percent by volume of the final admixture of the particles should be at least ten percent and may be as high as 50 percent.

By way of summary, the behind-the-casing fluid is an admixture formed of a high viscosity, gelled nonaqueous liquid slurry having an API fluid loss of less than 0.1 cc. after thirty minutes, and a select batch of particleform solids of graded size distribution, strength, chemical resistance and amount such that the behind-the-casing fluid will form a strong mobile seal of solid particles between the casing and wall of the borehole at a predetermined position. This seal will be at least two inches long and may be up to several feet in length, but must not be long enough to impede motion of casing. The seal prevents the flow of casing fluid or formation fluids from behind the casing to the well bore. This seal will be strong enough to withstand the large difference in fiuid pressure between the casing fluid and the fluid in the well bore which well bore fluid may be a gas. Although the seal or bridge of particles is very strong, it is important that the seal have a low unit shear strength. This prevents sticking of the casing and allows the bridge to reform or readjust itself to fit changes in borehole configuration or changes in formation hardness.

When using a filter sealing element to control the length of the particle seal, it may be possible at times to use a highly gelled, viscous nonaqueous fluid admixture containing a rather standard particle size distribution that builds a standard one or two inch bridge before becoming impermeable, but for many reasons previously set forth, it is highly desirable that the behind-the-casing fluid admixture be a highly gelled, high viscosity oil having three fluid loss and filter cake forming properties. First, the admixture is a high fluid loss, high filter cake forming mixture when exposed to cracks of a size greater than a predetermined size, for example, one-sixteenth inch; second, the admixture is a low control spurt loss fluid when exposed to openings less than the predetermined size; and third, the admixture is a very low fluid loss mixture when exposed to openings of the nature found in porous formations. The forementioned fluid loss properties are obtained by controlling the size distributions and relative concentrations of the particles in the admixture, and by designing the admixture to spurt from the annulus between the casing and borehole through a filter sealing element to form a particle seal which will move with the casing while sealing this annulus space.

The particles in the behind-the-casing fluid admixture are divided into a first and a second group with the first group being composed of large size particles in a narrow range and the second group being composed of small size particles small enough to pass through the openings between a layer of the particles in the first group. The second group of particles cover a wide range of sizes between colloidal size and a predetermined maximum size. The range of particle sizes covered by the second group is designed to quickly plug openings smaller than the predetermined maximum size particle in the second group.

A satisfactory test mixture contained 63 pounds of barites, pounds of fine mica, 9 pounds of 0.03 to 0.055 inch phenolic flash, pounds of 0.16 inch phenolic flash and 10 pounds of 0.25 inch phenolic flash mixed with a base liquid comprised of 303 pounds cutback road asphalt, 11 pounds of tall oil, 8 pounds of 50 percent sodium hydroxide solution, and one percent by weight of high melting asphalt particles. This mixture will have a Bro-okfield viscosity in excess of 20,000 centipoises, a gel strength of 65 pounds per hundred square feet after ten minutes, and will form a 3.75 inch particle seal in a standard smoothwalled slot tester. The particle seal will withstand pressure differentials exceeding 5000 p.s'.i. The mixture has a standard thirty minute API fluid loss of zero and after 19 hours loses lessthan 1 cc. At degrees Fahrenheit, the thirty minute fluid loss was 0.2 cc. and after 48 hours, 6.0 cc.

As stated previously, the behind-the-casing fluid admixture has a high viscosity and gel strength. In deep wells, this could preclude the exertion of full hydrostatic pressure against potentially productive zones or against the particle seal when moving the casing downward relatively rapidly; consequently, at such times it may be necessary to maintain a surface pressure at the top of the annulus between the casing and borehole. This positive surface pressure would overcome the drag effects of the high viscosity and gel strength of the behind-the-casing fluid and cause it to move with the casing.

In some cases, it will be desirable to have a third fluid disposed in the casing-borehole annulus above the behindthe-casing fluid admixture. This third fluid may be the same as that behind-the-casing fluid admixture except for the presence of group one particles and the fact that it may have a lower viscosity or gel strength. It is possible in deep wells to further grade the fluid in gel strength, viscosity and particle size and concentration as one nears the surface.

Although specific examples and illustrations have heretofore been given, it is to be understood that modifications and variations thereof will be apparent to those skilled in the art. Accordingly, the present invention is to be limited only in accordance with the appended claims.

I claim:

1. A method of drilling boreholes in the earth wherein a tubular casing is lowered behind an earth drill while the borehole is being drilled, which method comprises (a) drilling a borehole having a diameter at least onehalf inch larger than the diameter of said casing by progressively advancing said drill downwardly into said earth, said drill being connected to a tubular drill string;

(b) maintaining a circular cutting edge surrounding said casing and near the lower end of said casing, said cutting edge having an outside diameter substantially equal to the diameter of said borehole;

(c) advancing said casing into said borehole being drilled by said drill at a distance above said drill, said casing surrounding said drill string and having an internal diameter larger than said drill string;

((1) cutting inwardly extending projections from the walls of said borehole with said cutting edge as said cutting edge is moved downwardly with said casing;

(e) circulating a first drilling fluid adapted to carry drill cuttings to the surface of the earth through the interior of said drill string and the annular space between said drill string and said casing; and

(f) maintaining a flowable admixture of a gelled, highly viscous, nonaqueous liquid and particle-form solids of graded size in the annular space between the wall of said borehole and the exterior of said casing above the lower end of said casing, said solids consisting of a first group of large-sized particles covering a narrow size range and a second group of smaller-sized particles covering a broad range of particle sizes ranging from colloidal size to the size of the largest particles in said second group, said second group of particles being characterized by the fact that the average size of the particles in said first group is at least three times 0.75 the size of the largest size particles in said second group, and said admixture having an API fluid loss of less than 1 cc. in thirty minutes 17 at 130 degrees Fahrenheit and having a viscosity of at least 1,000 centipoises.

2. The method of claim 1 wherein the first drilling fluid has a specific gravity less than the specific gravity of the flowable admixture behind the casing.

3. The method of claim 1 wherein the particle-form solids are present in an amount of about to 50 percent by volume of the admixture.

4. The method of claim 1 wherein the percent by volume of the first group of particles is at least three times the percent by volume of all the particles in the second group having a size greater than 0.5 the size of the largest size particles in the second group.

5. The method of claim 1 wherein the admixture has a viscosity of at least 10,000 centipoises.

6. The method of claim 1 wherein the gel strength of the admixture is between 10 and 100 pounds per hundred square feet after ten minutes.

7. The method in accordance with claim 1 wherein the casing is intermittently advanced after advancement of the drill bit.

8. A method of drilling boreholes in the earth wherein a tubular casing is lowered behind an earth drill while the borehole is being drilled, which method comprises (a) drilling a borehole larger than the diameter of said casing by progressively advancing said drill downwardly into said earth, said drill being connected to a tubular drill string;

(b) advancing said easing into said borehole being drilled by said drill at a distance above said drill, said casing surrounding said drill string and having an internal diameter larger than said string;

(c) circulating a first drilling fluid adapted to carry drill cuttings to the surface of the earth through the interior of said drill string and the annular space between said drill string; and

(d) maintaining a seal consisting of an unconsolidated,

fluid-impermeable mass of particle-form solids between the Wall of said borehole and said casing, said seal following the movement of said casing as said casing is advanced into said borehole and preventing the flow of fluids between the annular space between the wall of said borehole and said casing and the inten'or of said casing.

9. The method in accordance with claim 8 wherein below the particle seal and near the lower end of said casing, there is maintained a circular cutting edge having an outside diameter substantially equal to the diameter of said borehole, and said cutting edge is moved downwardly with said casing to cut inwardly extending projections from the walls of said borehole.

10. The method of claim 8 wherein the first drilling fluid has a specific gravity less than the specific gravity of the flowable admixture behind the casing.

11. The method in accordance with claim 8 wherein the casing is intermittently advanced along with the advancement of the drill bit.

12. Apparatus for drilling a borehole in the earth and running a casing behind the drill bit comprising (a) a drill bit mounted on the lower end of a tubular drill string;

(b) a tubular casing disposed above said drill bit and surrounding said drill string, said tubular casing having an internal diameter great enough to permit removal of said drill bit and said drill string through said tubular casing;

(c) a circular cutting edge having an outside diameter substantially equal to the diameter of said borehole, said cutting edge being near the lower end of said casing and adapted to move with said casing and to cut inwardly extending projections from the walls of said borehole; and

(d) a lateral projection surrounding said casing near the lower end of said casing above said cutting edge, said lateral projection having an outside diameter at 18 least 0.25 inch greater than the outside diameter of said casing and at least as great as the diameter of said borehole minus 0.1 inch and an unconsolidated, fluid-impermeable mass of particle-form solids deposited on said lateral projection. 13. Apparatus for drilling a borehole in the earth and running a casing behind the drill bit comprising (a) a drill bit mounted on the lower end of a tubular 'drill string; (b) a tubular casing disposed above said drill bit and surrounding said drill string, said tubular casing having an internal diameter great enough to permit removal of said drill bit and said drill string through said tubular casing; (c) a circular cutting edge having an outside diameter substantially equal to the diameter of said borehole, 7

said cutting edge being near the lower end of said casing and adapted to move with said casing and to cut inwardly extending projections from the walls of said borehole; and

(d) a filter sealing element surrounding said casing near the lower end of said casing above said cutting edge, said filter sealing element having a lateral projection surrounding the lower end thereof, said lateral projection having an outside diameter greater than the outside diameter of the remainder of said filter sealing element and at least as great as the diameter of said borehole minue 0.1 inch, said filter sealing element having flow passages for a distance of at least two inches above said lateral projection, and said flow passages being adapted to pass solid particles of a predetermined size.

14. The apparatus of claim 13 wherein the lateral projection is resilient and has a free expanded diameter greater than the diameter of the borehole.

15. The apparatus of claim 13 wherein there is a pressure differential actuated sealing means adapted to prevent downward flow of liquid in the annular space between the casing and wall of the borehole whenever downward flow of said liquid would otherwise exceed a predetermined rate, said pressure differential actuated means being located above the filter sealing element.

16. The apparatus of claim 13 wherein there are at least two sealing elements longitudinally spaced from one another.

' 17. The apparatus in accordance with claim 13 wherein the drill bit comprises a main bit body, rotary pilot drilling means protruding from the bottom of said bit body, said pilot drilling means having a width less than the width of said bit body and a vertical central axis offset from the vertical central axis of said bit body, a noncu-tting surface sloping outwardly from the center of said bit body and forwardly with relation to the direction of the main drilling force of said pilot drilling means, said noncutting surface being located on the half of said pilot drilling means furthest from said vertical central axis of said bit body and adapted to urge said pilot drilling means inwardly toward the center of the borehole being drilled, at least one first cutting means on that portion of said pilot drilling means nearest the central axis of said main bit body extending from adjacent said vertical central axis of said pilot drilling means to the inner edge of said pilot drilling means, said at least one first cutting means being adapted to cut a cylindrical pilot hole whose circumference accommodates the outermost extension of said noncutting surface when said vertical central axis of said pilot drilling means coincides with the central axis of said borehole, at least one second cutting means located on the side of the main bit body opposite the pilot drilling means and above said at least one first cutting means of said pilot drilling means, said at least one second cutting means trailing said noncutting surface by a distance of between 0.15 to 1.5 times the maximum width of said bit and adapted to cut an annular section of earth immediately surrounding the pilot hole drilled by said pilot 19 drilling means and trailing said pilot hole with respect to the direction of penetration of said bit, and said drill bit being adapted to cut a borehole having a diameter at least one-tenth larger than the outside diameter of the tubular casing and being small enough to pass through said tubular casing.

18. An improved fluid for use behind the casing when running casing while drilling and for forming a movable seal of solid particles near the lower end of said casing, which fluid comprises a flowable admixture of a gelled nonaqueous liquid and particle-form solids, said solids consisting of a first group of large-size particles covering a narrow size range and a second group of smaller size particle covering a broad range of particle sizes ranging from colloidal size to the size of the largest size particles in said second group, said second group of particles being characterizcd by the fact that the average size of the particles in said first group is at least three times 0.75 the size of the largest size particles in said second group, and said admixture having an API fluid loss of less than 1 cc. in thirty minutes at 130 degrees Fahrenheit and having a viscosity of at least 1,000 centipoises.

19. The improved fluid of claim 18 wherein the particle-form solids are present in an amount of about to 50 percent by volume of the admixture.

20. The improved fluid of claim 18 wherein the percent by volume of the first group of particles is at least three times the percent by volume of all the particles in the second group having a size greater than 0.5 the size of the largest size particles in the second group.

21. The improved fluid of claim 18 wherein the admixture has a viscosity of at least 10,000 centipoises.

22. The improved fluid of claim 18 wherein the gel strength of the admixture is between 10' and 100 pounds per hundred square feet after ten minutes.

23 Apparatus for drilling a borehole in the earth and running a casing behind the drill bit comprising (a) a drill bit mounted on the lower end of a tubular drill string;

(b) a tubular casing disposed above said drill bit and surrounding said drill string, said tubular casing having an internal diameter great enough to permit removal of said drill bit and said drill string through said tubular casing; and

(c) a filter sealing element surrounding said casing near the lower end of said casing, said filter sealing element having a lateral projection surrounding the lower end thereof, said lateral projection having an outside diameter greaterthan the outside diameter of the remainder of said filter sealing element and at least as great as the diameter of said borehole minus 0.1 inch, said filter sealing element having flow passages for a distance of at least two inches above said lateral projection, and said flow passages being adapted to pass solid particles of a predetermined size.

24. The apparatus of claim 23 wherein the lateral projection is resilient and has a free expanded diameter greater than the diameter of the borehole.

25. The apparatus of claim 23 wherein there is a pressure differential actuated sealing means adapted to prevent downward flow of liquid in the annular space between the casing and wall of the borehole whenever downward flow of said liquid would otherwise exceed a predetermined rate, said pressure differential actuated means being located above the filter sealing element.

26. The apparatus of claim 23 wherein there are at least two sealing elements longitudinally spaced from one another.

27. The apparatus in accordance with claim 23 wherein the drill bit comprises a main bit body, rotary pilot drilling means protruding from the bottom of said bit body, said pilot drilling means having a width less than the width of said bit body and a vertical central axis offset from the vertical central axis of said bit body, a noncutting surface sloping outwardly from the center of said bit body and forwardly with relation to the direction of the main drilling force of said pilot drilling means, said noncutting surface being located on the half of said pilot drilling means furthest from said vertical central axis of said bit body and adapted to urge said pilot drilling means inwardly toward the center of the borehole being drilled, at least one first cutting means on that portion of said pilot drilling means nearest the central axis of said main bit body extending from adjacent said vertical central axis of said pilot drilling means to the inner edge of said pilot drilling means, said at least one first cutting means being adapted to cut a cylindrical pilot hole whose circumference accommodates the outermost extension of said noncutting surface when said vertical central axis of said pilot drilling means coincides with the central axis of said borehole, at least one second cutting means located on the side of the main bit body opposite the pilot drilling means and above said at least one first cutting means of said pilot drilling means, said at least one second cutting means trailing said noncutting surface by a distance of betwen 0.15 to 1.5 times the maximum width of said bit and adapted to cut an annular section of earth immediately surrounding the pilot hole drilled by said pilot drilling means and trailing said pilot hole with respect to the direction of penetration of said bit, and said drill bit being adapted to cut a borehole having a diameter at least one-tenth larger than the outside diameter of the tubular casing and being small enough to pass through said tubular casing.

28. A method of drilling boreholes in the earth wherein a tubular casing is lowered behind an earth drill while the borehole is being drilled, which method comprises (a) drilling a borehole larger than the diameter of said casing by progressively advancing said drill downwardly into said earth, said drill being connected to a tubular drill string;

(b) advancing said casing into said borehole being drilled by said drill at a distance above said drill, said casing surrounding said drill string and having an internal diameter larger than said drill string;

(c) circulating a first drilling fluid adapted to carry drill cuttings to the surface of the earth through the interior of said drill string and the annular space between said drill string and said casing; and

(d) maintaining a flowable admixture of a gelled, highly viscous, nonaqueous liquid and particle-form solids of graded size in the annular space between the wall of said borehole and the exterior of said casing above the lower end of said casing, said solids consisting of a first group of large-sized particles covering a narrow size range and a second group of smaller-sized particles covering a broad range of particle sizes ranging from colloidal size to the size of the largest particles in said second group, said second group of particles being characterized by the fact that the average size of the particles in said first group is at least three times 0.75 the size of the largest size particles in said second group, and said admixture having an API fluid loss of less than 1 co. in thirty minutes at degrees Fahrenheit and having a viscosity of at least 1,000 centipo ses.

29. The method of claim 28 wherein the first drilling fluid has a specific gravity less than the specifiic gravity of the flowable admixture behind the casing.

30. The method of claim 28 wherein the particle-form solids are present in an amount of about 10 to 50 percent by volume of the admixture.

31. The method of claim 28 wherein the percent by volume of the first group of particles is at least three times the percent by volume of all the particles in the second group having a size greater than 0.5 the size of the largest size particles in the second group.

32. The method of claim 28 wherein the admixture has a viscosity of at least 10,000 centiposes.

33. The method of claim 28 wherein the gel strength of the admixture is between 10 and 100 pounds per hundred quare feet after ten minutes.

34. The method in accordance with claim 28 wherein the casing is intermittently advanced after advancement of the drill bit.

References Cited UNITED STATES PATENTS Swan 17'5-65 X Sublin 175-39 8 X Minuth 175-72 Kennedye 166-241 Asketh 166-242 Bergstrom 17'5-171 X Williams 17-171 X CHARLES E. OOONNELL, Primary Examiner.

1/1918 Pickin 175-334 10 R. E. FAVREA-U, Assistant Examiner. 

8. A METHOD OF DRILLING BOREHOLES IN THE EARTH WHEREIN A TUBULAR CASING IS LOWERED BEHIND AN EARTH DRILL WHILE THE BOREHOLE IS BEING DRILLED, WHICH METHOD COMPRISES (A) DRILLING A BOREHOLE LARGER THAN THE DIAMETER OF SAID CASING BY PROGRESSIVELY ADVANCING SAID DRILL DOWNWARDLY INTO SAID EARTH, SAID DRILL BEING CONNECTED TO A TUBULAR DRILL STRING; (B) ADVANCING SAID CASING INTO SAID BOREHOLE BEING DRILLED BY SAID DRILL AT A DISTANCE ABOVE SAID DRILL, SAID CASING SURROUNDING SAID DRILL STRING AND HAVING AN INTERNAL DIAMETER LARGER THAN SAID STRING; (C) CIRCULATING A FIRST DRILLING FLUID ADAPTED TO CARRY DRILL CUTTINGS TO THE SURFACE OF THE EARTH THROUGH THE INTERIOR OF SAID DRILL STRING AND THE ANNULAR SPACE BETWEEN SAID DRILL STRING; AND (D) MAINTAINING A SEAL CONSISTING OF AN UNCONSOLIDATED, FLUID-IMPERMEABLE MASS OF PARTICLE-FORM SOLIDS BETWEEN THE WALL OF SAID BOREHOLE AND SAID CASING, SAID SEAL FOLLOWING THE MOVEMENT OF SAID CASING AS SAID CASING IS ADVANCED INTO SAID BOREHOLE AND PREVENTING THE FLOW OF FLUIDS BETWEEN THE ANNULAR SPACE BETWEEN THE WALL OF SAID BOREHOLE AND SAID CASING AND THE INTERIOR OF SAID CASING. 